The system and method disclosed herein relate generally to controlling liquid level in a vessel and, more specifically, to predictive controlling of water level in a drum type boiler in a power plant.
Drum type boilers are the most commonly used boilers in power generation plants. In the power generation industry, drum level trips due to ineffective drum level control response during transient operations are a primary cause of disruption in power generation and lead to a huge loss of plant availability and revenue.
Control of drum water level is a challenging problem due to complicated dynamics of two-phase flows, presence of waves, unknown heat and pressure disturbances, and load demands. Maintaining water level within limits is critical, as exceeding the limits will lead to trips or damage the equipment. A drop in water level will cause thermal fatigue in the drum. An increase in water level significantly increases the possibility of water droplets entering a superheater/steam turbine and thus damaging the superheater/steam turbine.
Conventional approaches to control of water level typically include actuation of a feed water control valve that supplies water to the drum in reaction to observed changes in level and steam flowrate. However, such approaches to control water level are challenging for drum type boilers, especially during transient operating conditions due to inverse response exhibited by such systems.
In the conventional approach, the water level is controlled so as to attain a fixed water level set point. Moreover, the controller is a reactive type controller i.e. the controller does not anticipate the control action required. The controller also does not take into account the variation in drum states/conditions (water level, pressure, temperature, or the like) and predicted disturbances (thermal load and pressure variations). The conventional approach does not provide trip free operation of the plant or provide enough time for an operator to intervene if a need arises.
There is a need for a more effective technique for controlling a liquid level in a vessel, such as for example, predictive control of liquid level in a drum type boiler, especially during transient operating conditions.